Cellular phone handsets with integrated frequency modulation (FM) receivers available in the market today use the headphone wire as a FM antenna. This one meter long wire can serve as an efficient monopole antenna at FM radio frequencies in the 100 MHz range. As the use of wireless headphones becomes prevalent there is a high demand to integrate the FM antenna within the cellular phone. The antenna length that can be integrated into a typical handset is about 1/50th the FM wavelength making it lossy and inefficient. A typical integrated FM antenna can have 20 dB to 30 dB less gain than a headphone wire antenna. Thus, impedance matching to the antenna can help in transmitting and receiving signals using the antenna.
The impedance of an integrated FM antenna can depend heavily on the phone or platform it is integrated into. It can be affected by the proximity of metals and ground-planes around it in the phone, by the phone body and construction, and by human body effect when the phone is held or in proximity to the human body. Thus, the performance of the receiver varies due to these uncertainties.
Phones with integrated FM transmitters are not typically concerned with impedance matching the transmitter to the antenna because the transmitter can simply run at higher power to compensate for the mismatch loss. This is the technique utilized by existing solutions.